A technique for disposing optical elements, such as a waveguide, a coupler, and a diode, on a substrate is known. For instance, a technique for forming, on a substrate, a demultiplexer for a wavelength division multiplexing system, which includes multiple waveguides and a photodetector receiving light passing through each of the waveguides, is known (see Patent Document 1, for example). Also, there is known a technique for receiving light entered from a side surface of a substrate by forming a waveguide layer around a photodetector disposed on the substrate (see Patent Document 2, for example). Further, there is known a technique for receiving light emitted from a light emitter such as a laser diode disposed on a substrate, by a photodetector through an optical waveguide and a ring resonator (see Patent Document 3, for example).
However, in a light source in which both a light emitter and a photodetector are disposed on a substrate, an end of an optical waveguide for transmitting light emitted from the light emitter to the photodetector is not formed at an end surface of the substrate. Accordingly, it is difficult to inspect, at a time of shipping or the like, the photodetector by inputting light to the photodetector.
The following is reference documents:    [Patent Document 1] United States Patent Application Publication No. 2015/0117811,    [Patent Document 2] United States Patent Application Publication No. 2002/0118906,    [Patent Document 3] United States Patent Application Publication No. 2013/0094527.